The long term goal of this proposal is to gain insight into the mechanisms underlying the generation of autoantibodies in systemic lupus erythematosis (SLE), a major human autoimmune disease. The predominant and pathogenic anti-self response in these autoimmune patients consists of autoantibodies to nuclear components, including DNA. The origin of these autoantibodies remains enigmatic, but antibodies with similar binding activity are expressed by the normal B cell repertoire (natural autoantibodies). Compared to naturally occurring anti-DNA autoantibodies, lupus autoantibodies of similar specificity are "affinity mature", i.e., they are somatically mutated and antigen-selected. In addition, while the former are IgM, the latter are in general IgG, suggesting that class switching, a crucial mechanism in the maturation of any antibody response, is also important in the generation of autoantibodies in SLE. In class switching to IgG, the constant (C) region of the mu H chain is replaced by Cgamma region, resulting in the acquisition of novel biological activities, including the ability to pass into the extravascular space, and, therefore, in the case of autoantibodies to produce tissue damage. IgG-switched B cells are numerous in the circulation of SLE patients, and IgG accounts for the majority of the pathogenic autoantibodies in these patients. We formally argue here that class switching to IgG occurs more frequently and more effectively in lupus mu+ B cells than in normal mu" B cells. We further argue that this enhanced IgM->IgG switching results from a higher expression of CD40L by lupus T and B cells, as well as from a higher switching propensity of the B cells of these patients, due to a polymorphism of the Cgamma gene promoter or switch regions, and/or to dysregulation of the CD30-dependent mechanism that, as we have recently shown, physiologically dampens IgG-inducing stimuli. To test our hypothesis, we propose: (i) to study CD40L expression in SLE B cells, and their capacity to promote switching to IgG hypothesis, we propose: (i) to study CD40L expression in SLE B cells, and their capacity to promote switching to IgG using our unique in vitro human monoclonal (CL-01) IgM+ IgD+ B cell system; (ii) to analyze the in vitro spontaneous and CD40:CD40-induced IgG class switching in SLE IgM+ IgD+ B cells; (iii) to analyze the regulatory regions upstream of the Cgamma1, Cgamma2, Cgamma3, and Cgamma4 genes in SLE patients, their family members, and for comparison healthy subjects; and finally, (iv) to analyze the down-regulation of the CD40-mediated Ig class switching by CD30, another cell surface molecule of the TNFR family, and the interference with this mechanism by soluble CD30. The proposed experiments should further our understanding of the means that lead to Ig class switching and generation of IgG autoantibodies in lupus, and may help design specific means of therapeutic intervention.